The overall aim of this proposal is to assign functional significance to specific structural regions of the coagulation factor IX. These studies have arisen as an outgrowth of work done in the last cycle of this program project grant. Using the same techniques that successfully identified the endothelial cell factor IX receptor as collagen type IV, we have identified a chimeric factor IX molecular that does not bind to the platelet factor IX binding site. We hypothesize that even if factor IX is otherwise fully active, factor IX that does not bind to platelets will be ineffective in a physiologic setting. We propose studies to understand the role of the platelet factor IX binding site by using this chimera in both in vitro studies and in vivo studies in hemophilic dogs and in a hemophilia B mouse strain developed in the last cycle of this program studies in hemophilic dogs and in hemophilia B mouse strain developed in the last cycle of this program project grant. We are also planning to examine the physiologic consequence of mutating Arg338 in factor IX to Leu. This mutation increases factor IXa activity at least 3 fold. This residue is coded for by a CG mutational hot spot, yet not mutations have been reported at this site. This is especially remarkable since 16 hot spots in factor IX account for 40% of all reported mutations. We hypothesize that mutations have been reported because mutation at that site do not cause hemophilia but rather are thrombogenic. We will test this hypothesis in hemophilia B dogs and mice. We are proposed studies to understand the residues of factor IX that are involved in substrate recognition and cleavage. In the last cycle of this grant, we established that, in the absence of factor VIIIa, porcine factor IXa has higher activity toward human factor X than does wild type factor IXa. We will isolate the residues responsible for this higher activity using a human-porcine chimera and point mutations. Also, we will examine the interactions of the Kunitz inhibitor protease nexin II with factor IXa to define residues involved in the extended binding site of factor IXa. Finally, we plan to define a binding site for factor VIIIa using a peptide from factor VIII that we have shown to bind to factor IXa and inhibit its activity. We will crosslink this peptide to factor IXa and isolate peptides of factor IXa to which it is bound. Overall, these studies will allow us to define specific structural regions of factor IX that are involved in a number of important physiologic functions.